The present invention relates to radiation measuring devices and in particular to an analog ratemeter circuit for use in conjunction with a Geiger-Mueller tube for detecting and measuring ionizing radiation levels.
A common method used to detect and measure ionizing radiation involves the use of a Geiger-Mueller (GM) tube which is adapted to produce electrical pulses at a rate that is proportional to the amount of radiation present. Geiger-Mueller tubes are characterized, however, by a dead time period which follows the production of each pulse, during which period the GM tube is unable to produce another pulse. The dead time characteristic of a GM tube therefore effects the accuracy of the measurement in high intensity radiation fields.
To improve the high field accuracy of their radiation detectors, most manufacturers have either reduced the sensitivity of the GM tube, which adversely effects low field response, or have resorted to the use of two separate GM tubes, a low sensitivity tube for high fields and a high sensitivity tube for low fields. In addition, various circuits have been proposed for compensating for the dead time of a GM tube. However, these circuits have generally proven to be quite complex and not readily compatible with the analog-type ratemeters most widely used today in survey instruments.
Accordingly, it is the primary object of the present invention to provide a coincidence loss correction circuit which compensates for GM tube dead time and is compatible with analog ratemeter circuits.
In addition, it is an object of the present invention to provide such an analog coincidence loss correction circuit which substantially increases the high field accuracy of the radiation measuring device and yet is relatively simple in design and inexpensive to manufacture.
In general, the coincidence loss correction circuit according to the present invention initially differentiates the electrical output pulses from the GM tube and then provides the signal to a retriggerable one-shot multivibrator circuit. In response to the receipt of a pulse from the GM tube, the multivibrator circuit is adapted to produce a square wave output pulse having a preset duration greater than the dead time of the GM tube. If a GM pulse occurs after the dead time of the Geiger tube but before the termination of the output pulse from the multivibrator circuit, the multivibrator circuit will be retriggered to extend the output pulse for an additional fixed duration.
The output from the multivibrator circuit is provided to the input of a modified charge pump ratemeter circuit which includes a charge circuit for producing an input current proportional to the pulse rate of the output from the multivibrator circuit which flows through one of a plurality of range resistors to thereby produce a corresponding output voltage that is read by an analog meter. In addition, the pulse output signal from the multivibrator circuit is also utilized to control the conductive state of an analog switch connected between an integrating capacitor and the range resistors to control the discharge of the capacitor. As will be seen, this effectively serves to disable the "time" measurement in the ratemeter circuit during those intervals when GM pulses are not being counted so that the meter provides a true measurement of the Geiger tube pulse rate compensated for tube dead time.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following drawing: